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Thermal tuning of infrared resonant absorbers based on hybrid gold-VO2
nanostructures
Hasan Kocer, Serkan Butun, Berker Banar, Kevin Wang, Sefaatttin Tongay, Junqiao Wu, and Koray Aydin
Citation: Applied Physics Letters 106, 161104 (2015); doi: 10.1063/1.4918938
View online: http://dx.doi.org/10.1063/1.4918938
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/16?ver=pdfcov
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APPLIED PHYSICS LETTERS 106, 161104 (2015)
Thermal tuning of infrared resonant absorbers based on hybrid gold-VO2
nanostructures
Hasan Kocer,1,2,a) Serkan Butun,1,a) Berker Banar,1,3 Kevin Wang,4 Sefaatttin Tongay,5
Junqiao Wu,4 and Koray Aydin1,b)
1
Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois
60208, USA
2
Department of Electrical Engineering, Turkish Military Academy, 06654 Ankara, Turkey
3
Department of Electrical and Electronics Engineering, Bilkent University, 06800 Bilkent, Ankara, Turkey
4
Department of Materials Science and Engineering, University of California Berkeley, Berkeley, California
94720, USA
5
School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287,
USA
(Received 17 February 2015; accepted 12 April 2015; published online 22 April 2015)
Resonant absorbers based on plasmonic materials, metamaterials, and thin films enable spectrally
selective absorption filters, where absorption is maximized at the resonance wavelength. By
controlling the geometrical parameters of nano/microstructures and materials’ refractive indices,
resonant absorbers are designed to operate at wide range of wavelengths for applications including
absorption filters, thermal emitters, thermophotovoltaic devices, and sensors. However, once
resonant absorbers are fabricated, it is rather challenging to control and tune the spectral absorption
response. Here, we propose and demonstrate thermally tunable infrared resonant absorbers using
hybrid gold-vanadium dioxide (VO2) nanostructure arrays. Absorption intensity is tuned from 90%
to 20% and 96% to 32% using hybrid gold-VO2 nanowire and nanodisc arrays, respectively, by
heating up the absorbers above the phase transition temperature of VO2 (68 ! C). Phase change
materials such as VO2 deliver useful means of altering optical properties as a function of temperature. Absorbers with tunable spectral response can find applications in sensor and detector applications, in which external stimulus such as heat, electrical signal, or light results in a change in the
C 2015 AIP Publishing LLC.
absorption spectrum and intensity. V
[http://dx.doi.org/10.1063/1.4918938]
Electromagnetic absorbers based on structured surfaces
including metamaterial and plasmonic nanostructures have
received burgeoning amount of attention in recent years and
have enabled spectrally selective absorption over microwave,1 terahertz,1 infrared (IR),2–6 and visible7–9 bands of
electromagnetic spectrum. In particular, controlling and
manipulating the spectral absorption properties of materials
in the IR range are an active area of research and could enable advances in applications such as target recognition, biochemical sensing, camouflage, IR signature mimicry, imaging, sensors, IR labelling, and wavelength selective IR sources.10,11 Having a tunable resonance response for IR
wavelength range is a desired feature for thermal emitters,12
thermophotovoltaic cells,2 as well as plasmonic scatters.13–15
In this study, we propose and demonstrate intensitytunable short-wavelength infrared nanostructured resonant
absorbers by utilizing a phase change material, vanadium
dioxide (VO2). VO2 undergoes a structural transition from
an insulating phase to a metallic phase at the transition temperature of 68 ! C. This reversible phase change occurs on a
sub-picosecond timescale.16,17 Around the phase transition
temperature, metallic VO2 (m-VO2) islands occurs inside the
insulating VO2 (i-VO2) and as the temperature increases, the
a)
H. Kocer and S. Butun contributed equally to this work.
Author to whom correspondence should be addressed. Electronic mail:
[email protected]
b)
0003-6951/2015/106(16)/161104/4/$30.00
entire VO2 film becomes metallic. The phase transition of
VO2 is mediated by heating the absorber device over the
transition temperature.
We theoretically and experimentally investigated two
different absorbers, (i) Au - VO2 periodic gratings and (ii)
Au cylinders embedded in VO2. Our two hybrid gold-VO2
nanostructured absorber designs are schematically depicted
in Figs. 1(a) and 1(b). While the former exploits discrete
VO2 stripes embedded in a gold matrix, the latter utilizes a
2D square lattice of gold cylinders embedded in VO2. We
will refer to these two different designs as “g-design” (Fig.
1(a)) and “c-design” (Fig. 1(b)) throughout this paper. Each
structure has an optically thick Au layer (100 nm) as a backside reflector to ensure that there is no light transmission. In
addition, samples are illuminated from the sapphire side.
Our proposed tunable absorbers are fabricated on epitaxially grown VO2 layers on double side polished sapphire substrate using conventional e-beam lithography and deposition
techniques. The details of the fabrication methods can be
found in supplementary material.18 The scanning electron
microscope (SEM) images of fabricated g-design and
c-design structures are shown in Figs. 1(c) and 1(d), respectively. Note that the deposition of Au layer was conducted
after patterning the VO2 layer. Therefore, SEM images in
Figs. 1(c) and 1(d) depict the topography of the gold surface
which follows the underlying VO2 pattern. This is clearly
visible in the cross-sectional focused ion beam (FIB) image
106, 161104-1
C 2015 AIP Publishing LLC
V
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161104-2
Kocer et al.
Appl. Phys. Lett. 106, 161104 (2015)
FIG. 1. Tunable IR absorber designs.
The schematic representations of grating (a) and cylinder (b) hybrid goldVO2 design, respectively. Relevant
design parameters are indicated on the
figures. (c) and (d) Scanning electron
microscope images of the grating and
cylindrical devices illustrated in (a)
and (b), respectively. Scale bars correspond to 2 lm. Both of the images are
acquired subsequent to gold deposition. (e) Focused ion beam crosssectional image of the grating design.
The scale bar corresponds to 500 nm.
The sample tilt is 45! .
given in Fig. 1(e). The actual width of the VO2 stripes in
g-design (and diameter of the cylinders in c-design) turns out
to be narrower than those measured in the SEM images,
which is taken in to account in our numerical calculations.
For a profound analysis of structures, we have performed full field finite difference time domain (FDTD) simulations using a commercial grade solver Lumerical FDTD.19
The experimental refractive index data of VO2 reported by
Dicken et al.16 were utilized in FDTD simulations. The total
absorption in the hybrid gold-VO2 structure is calculated by
1–R-T, where R is the total reflection at the sapphire side and
T is the transmission which is zero due to the optically thick
gold mirror at the back side. Therefore, the total absorption
can be calculated as 1–R in both measurements and simulations. The spectral reflectivity measurements were performed
by a Fourier transform IR (FTIR) spectroscopy coupled with
a heating stage and IR microscope. Please refer to supplementary material18 for detailed explanation of the methods
used for measurements and simulations. Simulated and
measured absorption spectra for the hybrid Au-VO2 wiregratings of periodicity, K, 1 lm for polarization along and
parallel to stripes are shown in Fig. 2. As for any 1D grating
structure, we observe an optical resonance when electric field
is perpendicular to stripes. FDTD simulations revealed two
distinct resonances at 1.7 and 2.6 lm (Fig. 2(a) blue curve).
The first resonance at 1.7 lm is at about neff " K which is the
lattice mode of the gold grating, where neff is the effective refractive index. Note that VO2 stripes between the gold ridges
make the effective refractive index slightly larger than refractive index of sapphire. The second mode at 2.6 lm is the
localized mode within the grooves of the underlying gold
grating where VO2 fills. The FTIR spectroscopy measurements (Fig. 2(c) blue curve) confirm this phenomenon with a
broadening of the first and a slight blue shift of the second
resonance wavelengths. The broadening of the first resonance is due the high numerical aperture of the illumination
objective (0.4) which effectively illuminates the sample with
a broad range of in-plane momentum. We attribute the slight
blue shift of the second resonance to poor fit of readily
available index data of VO2 to our samples because this resonance is highly sensitive to the refractive index of VO2.
Whereas the dependence of the lattice resonance to the refractive index of VO2 is minor. As expected, there is no resonance observed for the other polarization in neither
simulations nor measurements (Figs. 2(b) and 2(d)). We
additionally note that due to imperfections aroused during
the fabrication, we have a further mismatch between the
measured and simulated absorption spectra in terms of amplitude especially towards the long end of the spectrum.
The extraordinary behavior of the IR absorber occurs
when we heat the sample above the critical phase change
temperature (68 ! C). At an elevated temperature above the
critical point (120 ! C in our measurements), VO2 becomes
metallic, which substantially alters the refractive index compared to the insulating phase below the critical point. The
hybrid gold-VO2 structure at high temperatures behaves like
a continuous reflective surface. As a result, the absorption is
dramatically suppressed (Figs. 2(a) and 2(c) red curves).
Here, we theoretically and experimentally demonstrate an intensity tunable absorber between 2 lm and 3 lm such that
absorption can be either suppressed or enhanced in a positive
dynamic range depending on the phase of the VO2. 9-fold
and 4.5-fold increase at the resonance in the absorption intensity is obtained theoretically and experimentally, respectively, when the polarization of incident light is perpendicular
to the grating.
According to Kirchhoff’s law of thermal radiation, the
emissivity, E, of a material is equal to its absorptivity, A, at
thermal equilibrium.20 If the absorptivity, therefore emissivity decreases as the temperature rises, the thermochromic
structure has a positive dynamic range which is desired for
IR signature reduction. Opposite thermal behavior has negative dynamic range that is suitable for smart windows and
space applications.21 Although, we have not investigated
reduced thermal emission with increased temperature, our
tunable absorber structure can also be employed as an intensity tunable thermal emitter and has great potential for reducing IR radiation due to increased temperature.
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Appl. Phys. Lett. 106, 161104 (2015)
FIG. 2. The absorption characteristics
of the grating design tunable IR
absorber. (a) and (b) Simulated spectral absorption curves of gold/i-VO2
and gold/m-VO2 composite structures,
at perpendicular and parallel polarization relative to grating lines, respectively. Insets: Direction of the electric
field vector in relation to the unit cell
used in calculations. (c) and (d)
Measured spectral absorption curves of
gold/i-VO2 and gold/m-VO2 composite
structures, at perpendicular and parallel polarization relative to grating
lines, respectively. Insets: Direction of
the electric field vector in relation to
the SEM images of the respective
structure. In all graphs, blue curve
indicates the i-VO2 (insulator state,
room temperature) and the red curve
indicates the m-VO2 (metallic state,
high temperature).
For certain applications, polarization-independent
absorption characteristics are required; therefore, we introduce our second design based on cylindrical gold nanodiscs
embedded in VO2 film. Due to symmetry of the square lattice, absorption spectra are independent of the incident electric field polarization. Simulated and measured absorption
spectra for c-design are plotted in Figs. 3(a) and 3(b), respectively. Here, the absorption enhancement is approximately
4-fold and 3-fold in simulations and measurements, respectively, for a randomly polarized illumination. Furthermore,
simulated (Fig. 3(a)) and measured (Fig. 3(b)) results of our
c-design are in fairly good agreement. The similar arguments
in g-design IR absorber also apply here to lattice and localized modes. Moreover, structural inhomogeneity of the VO2
film and slight variation in heat conductivity may as well
result for minor discrepancies in measured and simulated
spectra.
Overall, with both wire and nanodisk Au arrays, we
have the ability to control the absorption intensity actively
with a high contrast ratio and high positive dynamic range
by changing the operation temperature of the device. At low
temperatures, VO2 shows insulator characteristics and in
both g-design and c-design cases, and metal (Au)–insulator
(VO2) nanostructured design creates sharp resonances. At
high temperatures, since the VO2 shows metallic characteristics, the material becomes fully reflective and resonant
behavior has not been observed. Therefore, the capability of
actively controlling the design to switch from one phase to
another allows turning the resonance on and off, as a result
of the phase transition of VO2.
In order to understand the electric field confinement and
absorption mechanism of our absorbers in two phases of
VO2 (insulator and metallic), we calculated the power
absorption map at the resonance wavelengths of 2.6 lm and
2.3 lm for g-design and c-design, respectively. Absorbed
power (Pabs) is the divergence of the Poynting vector for
non-magnetic materials and can easily be calculated using
the simple relation Pabs ¼ 12 xe2 jEj2 , where x is the angular
frequency, e2 is the imaginary part of the dielectric permittivity, and jEj is the absolute magnitude of the total electric
field.7,13 The results are shown in Fig. 4 where FDTD simulations were performed with an incident electric field parallel
to x-axis. These 3D absorption maps revealed that the
absorption mostly takes place in the VO2 layer. It is clear
FIG. 3. The absorption characteristics of the cylinder design tunable IR
absorber. (a) Simulated spectral absorption curves of gold/i-VO2 and gold/
m-VO2 composite structures, Inset: the unit cell used in calculations. (b)
Measured spectral absorption curves of gold/i-VO2 and gold/m-VO2 composite structures. Inset: the SEM images of the measured structure. In both
graphs, blue curve indicates i-VO2 (insulator state, room temperature) and
the red curve indicates m-VO2 (metallic state, high temperature).
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161104-4
Kocer et al.
Appl. Phys. Lett. 106, 161104 (2015)
FIG. 4. At 2.6 lm resonant wavelength, 3D power absorption (Pabs) map
of the g-design (a) at room temperature (i-VO2) and (b) at high temperature (m-VO2). At 2.3 lm resonant wavelength, 3D power absorption map
of the c-design (c) at room temperature (i-VO2) and (d) at high temperature
(m-VO2).
that the i-VO2 (Fig. 4(a)) has stronger absorption compared
to the m-VO2 (Fig. 4(b)). Similar behavior is evident for
c-design. The absorption is highest in the i-VO2 (Fig. 4(c))
between the gold cylinders along the polarization direction,
whereas the m-VO2 layer (Fig. 4(d)) has negligible power
absorption compared to the insulating phase. This vast
absorption contrast between the two phases of both-design
structures is originating from highly localized electric field
inside the i-VO2 film. In the insulating phase, in which VO2
behaves like an ordinary lossy dielectric. Therefore, resonance modes of the periodic metallic structure are available
for coupling. However, in the metallic phase of VO2, the
imaginary part of the dielectric permittivity is so high that
field penetration is minimal (see supplementary material.18)
Therefore, there is no resonance mode available and the incident light is mostly reflected back.
In conclusion, we demonstrated intensity tunable resonant absorber operating at the short-wavelength infrared
region by utilizing the phase transition of VO2 via thermal
stimulus. By heating up our devices to a temperature roughly
of >80 ! C, we were able to tune the absorption from 90% to
20% and from 96% to 32% for linearly polarized and unpolarized IR illumination, respectively. This kind of a device
finds its use in many practical applications that works in
short wave IR spectrum such as short-wave imaging
systems.
This research was supported by the Materials Research
Science and Engineering Center (NSF-MRSEC) (DMR-
1121262) of Northwestern University. K.A. acknowledges
financial support from the McCormick School of Engineering
and Applied Sciences at Northwestern University and partial
support from the AFOSR under Award No. FA9550-12-10280 and the Institute for Sustainability and Energy at
Northwestern (ISEN) through ISEN Booster Award. The
material preparation work at Berkeley was supported by a
NSF CAREER Award under Grant No. DMR-1055938. H.K.
was supported by The Scientific and Technological Research
Council of Turkey (TUBITAK) through a postdoctoral
research fellowship program. This research made use of the
NUANCE Center at Northwestern University, which was
supported by NSF-NSEC, NSF-MRSEC, Keck Foundation,
and the State of Illinois and the NUFAB cleanroom facility at
Northwestern University.
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